i_shared_lock is held for a very long time during vmtruncate() and causes
high scheduling latencies when truncating a file which is mmapped.  I've seen
100 milliseconds.

So turn it into a semaphore.  It nests inside mmap_sem.

This change is also needed by the shared pagetable patch, which needs to
unshare pte's on the vmtruncate path - lots of pagetable pages need to
be allocated and they are using __GFP_WAIT.

The patch also makes unmap_vma() static.